In recent years, positive temperature coefficient (PTC) polymer materials have been widely applied to self limiting heating cables, over-current protection devices e.g. thermistors, touch-control elements, and the like. Due to the extensive development, application and dissemination of electronic products, such as computers and peripheral equipments thereof, cellular phones, secondary rechargeable batteries, network interface boards/machines, modems and electronic facilities and so on, the need for over-current protection devices has remarkably increased. Particularly, the trend for present electronic products is to be light and/or accurate, and the characteristics of an over-current protection device made from PTC polymer materials are being utilized to help meet this trend.
PTC materials are conductive materials characterized by a sharp increase in resistivity upon reaching a switching temperature (Ts). If the jump in resistivity is sufficiently high, the resistivity effectively blocks the current and further heating of the material such that overheating of the material is prevented. One of the main benefits of PTC materials is that no additional electronic circuits are necessary in an article that includes a PTC material since the PTC material itself has a characteristic similar to electronic circuits. Moreover, upon cooling, the material resets itself. This jump in resistivity may oftentimes be referred to as the PTC amplitude and may be defined as the ratio of the maximum volume resistivity to the volume resistivity at room temperature (app. 23° C.). In many prior art embodiments, PTC polymer materials primarily have been prepared by the addition of conductive additives, such as carbon black and metal powders, to polymer materials. Crosslinking of a conductive polymer by chemicals or irradiation, or the addition of inorganic fillers or organic additives may also be employed to improve electrical stability.
As mentioned, carbon black is one material that has been used in PTC materials. Carbon black is one of the most frequently used conductive fillers for polymer based PTC materials. Some of the advantages of using carbon black as compared to conductive metal fillers include a lower cost price and a lower density. According to the prior art, medium-sized, low-structured carbon blacks yield PTC materials with a good balance between conductivity and PTC amplitude. Typically these carbon black based polymeric PTC materials are employed in circuit breakers and (resettable) fuses. Nevertheless, the use of these PTC materials as true heaters is limited due to the limited thermal conductivity of the “pure” carbon black based polymeric PTCs. Any heater needs to transfer the heat generated to another medium. The higher the thermal conductivity the better the heat transfer to the outside/the medium that is desired to be heated up.
In one prior art embodiment, carbon black is used to provide thermal conductivity. According to some prior art teachings, medium sized low-structured carbon blacks yield PTC materials with a good balance between conductivity and PTC amplitude. Typically these carbon black-based polymeric PTC materials are employed in circuit breakers and (resettable) fuses. Nevertheless, the use as of these PTC materials as true heaters is limited due to the limited thermal conductivity of “pure” carbon black-based polymeric PTCs. Heaters based on these PTC materials need to transfer the heat generated to another medium. The higher the thermal conductivity, the higher the heat transfer to the outside/another medium and the more effective the heater. In addition, a better heat conductivity can prevent or smoothen hot spots that can occur when the cross section of a material between the electrodes changes.
Alternative prior art embodiments utilize small graphite particles (i.e. having an average diameter less than 10 microns) as a conductive filler in PTC materials. However, small diameter graphite particles are more expensive due to the costs associated with making these small particles and the use of such small particles also is more difficult in a manufacturing environment due to health and environmental safety issues.
Accordingly, as may be seen, it would be beneficial to provide a PTC material that has improved thermal conductivity as compared to prior art PTC materials without the disadvantages associated with prior art materials.